The Big Spring #4 drillcore in Clayton County, northeast Iowa, was sampled for carbon isotopic values of micritic carbonates as part of a regional study of the chemostratigraphy of Ordovician units in the Upper Mississippi Valley. Starting out from baseline values in the Platteville Formation of around -2 o/oo VPDB, the d13C rises to a peak of near 0 o/oo before falling to values of -2.5 to -3 o/oo in the uppermost few meters of the unit. Moving up into the Decorah Formation, values rise to around +0.5 o/oo in the Spechts Ferry Member. Then they dip to -0.5 o/oo before rising again to about +1 o/oo in the Guttenberg Member. In the Ion Member, the d13C stabilizes out to roughly -0.5 o/oo, which continues upward through our sampled interval of the Dunleith Formation.